1. Technical Field
The present invention relates generally to a spintronics area and, more particularly, to spin-polarized electron sources and spin-polarized scanning tunneling microscopes incorporating the same.
2. Description of Related Art
Spintronics (a neologism for “spin-based electronics”), also known as magneto-electronics, is an emergent technology which exploits the quantum propensity of electrons to spin as well as making use of their charge states. The spin itself is manifested as a detectable weak magnetic energy state characterized as “spin-up” and “spin-down”. One proposed spintronic device is a spin-polarized scanning tunneling microscope tip (SP-STM tip) used in a spin-polarized scanning tunneling microscope (SP-STM). In SP-STM, the SP-STM tip and a sample separated by a vacuum gap (usually in a range from 0.1 to 10 nanometers) have spin-polarized surface electron states, and usually the SP-STM tip acts as a spin-polarized electron source and the sample correspondingly acts as a spin sensitive electron receiver. Based on the operation of the SP-STM, spin-dependent tunneling currents are recorded locally as a function of the position of the SP-STM tip to produce a spin-dependent image, whereby a surface magnetic structure of the sample can be obtained. More detailed information on the configuration and the operation of the SP-STM is taught in U.S. Pat. No. 4,985,627 entitled “Spin-Polarized Scanning Tunneling Microscope”, which is incorporated herein by reference. So far, there are two typical spin-polarized electron sources can be used as the SP-STM tip. Structures of the two spin-polarized electron sources will be described in detailed as follows.
Regarding one typical spin-polarized electron source, a multi-layer structure, in which cesium (Cs) layers and oxygen (O) layers are alternately laminated, is deposited on a surface of a p-type gallium arsenide (GaAs) layer, in order to produce a negative electron affinity. Electrons can be extracted from the surface of the GaAs layer by irradiating it with a circularly polarized laser beam having an energy substantially equal to the forbidden band of GaAs. However, in the band structure of the three-dimensional GaAs layer, a band for heavy holes and a band for light holes are degenerated in the valence band, and, therefore, a ratio of spin-down electrons to spin-up electrons is 3:1 because of the difference in transition probability when electrons are excited from these bands to the conduction band. For this reason, there is only a maximum polarization of 50% can be obtained.
As to another one typical spin-polarized electron source, a macroscopic tungsten tip, covered with a fine film (approximately 50 nanometers thick) of an insulating material, is provided. The insulating material, such as EuS, is ferromagnetic at a very low temperature. EuS, in particular, is a ferromagnetic insulator with a Curie temperature of 16.5 K (Kelvin). By raising the voltage in the tungsten tip to several kilovolts with respect to an anode and subjecting the tungsten tip to a magnetic field parallel thereto, electrons can be extracted from the fine film by field effect and acquire, on passing through the EuS layer, a spin polarization. A degree of polarization of up to about 86% can be obtained at an operation temperature of 9 K.
Generally, for practical spin-polarized electron sources, it is crucial to realize continuous and efficient emission of a spin-polarized electron current/beam at room temperature. However, the conventional spin-polarized electron sources, as mentioned above, clearly would not meet the requirement.
Therefore, what is needed is to provide a spin-polarized electron source, which can realize continuous and efficient emission of a spin-polarized electron current/beam at room temperature, and a spin-polarized scanning tunneling microscope incorporating such a spin-polarized electron source therein.